BZFlag: open-source, portable, online multi-player game 12 years ago
Probably my favourite game ever. Played mainly by die-hard experienced players. There's always talk of the game dying off, but still, here it is alive and reasonably well! The graphics would not impress anybody who is used to modern multiplayer games, but the gameplay is second to none. Easy to learn, difficult to master. And great fun to boot. Give it a try!